Well, far from I Know act 4 drops most i62 and i63 legends now, while act 3 will still drop some 59-61... That's the really big difference... and on long logs Act 4 will have more i63 lvls due to monsters lvls witch includes fast xp...
Don't think those stats are enough... My barb has bout 60k hp, 10kdef 900all res 70kdps, and still dies like paper on act 3... (1 lag crash or bot bugs for a while, and I'm dead)...
I'm ain't coming back there w/o nearly 100k hp/ 100k dps ^^ and 1k all res... well extra healths moobs there have 4m hp+-... So anything less then this is just effect-less... But if u have the gears for making as "easy" as act 3... yeah its fuckin worth...